In recent years, with the rapid development of wireless communication throughout the world, resources of radio frequency spectrum becomes more and more insufficient. In order to utilize the finite spectrum resources effectively, many wireless communication systems have adopted linear modulation which has a high rate of spectrum utilization. Since both the phase and amplitude of linear modulation signals carry useful information, any nonlinear amplification of these signals will increase bit error rate and disturb adjacent radio channels. These two adverse effects should be absolutely avoided when designing wireless communication systems, therefore the linear modulation signal must be amplified with high linearity.
A typical practice to increase the linearity of RF (Radio Frequency) power amplifier is to make the output power of the amplifier far below the 1 dB compression point of the amplifier, i.e., only a short relatively linear section of input/output characteristic of RF power amplifier is used to obtain a highly linear amplification. This method is called power back-off. However, this power back-off technique has the following obvious disadvantages: firstly, the improvement of linearity according to this method is at expense of power efficiency of the amplifier, which means that the power output capability of the amplifier is not fully utilized, therefore decreasing the power efficiency of the amplifier and increasing the cost; secondly, the low power efficiency means that most power is released in the form of heat, bringing a huge pressure on the ventilation and heat dispersion system of the whole base station; finally, an amplifier of low power efficiency needs an electrical power system with larger capacity, which is also an important factor for the increased cost of transmitter.
In order to increase the power efficiency of the RF power amplifier and decrease the cost of transmitter, a typical method is to make the power amplifier work in a state of lower linearity and higher power efficiency, then increase the linearity of the amplifier at outside through certain measures. These measures are called linearization technique.
Both the two techniques, feed-forward linearization and feedback linearization, have been widely used in all kinds of wireless communication systems. But both these two techniques have limitations: the main problem of the feed-forward technique lies in poor power efficiency, complicated structure and high cost of the RF power amplifier; while the feedback technique has the problem of unstable loop and limited linearization bandwidth. Therefore, neither of the above two techniques can offer highly linear amplification on wide frequency band with the power efficiency required by next generation mobile communication base station.
Another typical linearization technique is predistortion technique, which can obtain higher power efficiency than the feed-forward technique. Different from the feed-forward technique, this predistortion technique implements compensation before the signal amplification, i.e., the input signal is pre-distorted in advance in a way opposite to the nonlinear characteristic of the amplifier, so that the whole effect of the signal predistortion and nonlinear amplification approximates to the linear amplification. The predistortion technique can be classified into digital base band predistortion, analogue base band or intermediate frequency predistortion, RF predistortion, etc., among which, the adaptive digital base band predistortion technique based on digital signal processing has been developed rapidly in recent years. The usual digital base band predistortion technique stores predistortion compensation parameters that have been calculated beforehand in a 1-D predistortion Look-Up table (LUT), calculates an address according to the current amount of the input signals, and then corrects the input signals in a certain way after finding one or a group of corresponding predistortion compensation parameters in corresponding positions in the predistortion Look-Up table.
However, if without special measures, both the linearization bandwidth and linearization capacity that can be provided by the digital predistortion linearization method will be very limited. The reasons are: on one hand, this predistortion technique implicates an unavoidable paradox that, in order to compensate part of non-linearization, the amplifier is set in a more nonlinear state resulting in impossibility of compensation in some conditions, therefore the compensation effect has a close relation with back off of the amplifier and peak-to-average ratio of the input signal; on the other hand, the amplifier has memory effect which greatly influences the predistortion effect. With regard to time domain, when memory effect exists, the distortion characteristic of the amplifier will relate not only to the current input, but also to its previous input; with regard to frequency domain, when memory effect exists, the amplitude and phase of the nonlinear distortion component of the amplifier will vary along with the modulation frequency of the input signal, and such a varying distortion signal can not be totally compensated by the predistortion signal having constant amplitude and phase.
In the international application PCT/CN/00774 entitled “A Method and a System of Wideband Predistortion Linearization” which was filed on Oct. 31, 2002 by this inventor, a method for wideband predistortion linearization and a device thereof are provided, wherein, the compensation for amplifier's AM-PM distortion, i.e., non-linear phase distortion is realized in band (the “in band” means the compensation signal and original signal are always in the same frequency band); and the compensation for amplifier's AM-AM distortion (nonlinear amplitude distortion) and memory effect is realized out of band (the “out of band” means the frequency band where the compensation signal locates is far from the frequency band where the original signal locates, and these two signals do not overlap with each other). One the one hand, this method can compensate the memory effect of the amplifier and extend the linearization bandwidth of the same, on the other hand, since the aforementioned paradox of the conventional predistortion method can be avoided, this method enables the amplifier work in a state more approximating to 1 dB compression point, thereby further increasing the efficiency of the amplifier.
However, in this method, in order to compensate the AM-PM distortion, part of the predistortion has to be processed in band, so the system is required to provide base band I, Q signals respectively, which will be inconvenient in some application environments, especially in wireless communication field. The reasons are: compared with separated I, Q information streams in digital base band, in the case of radio frequency (RF) or intermediate frequency (IF), the I, Q signals carrying the I, Q information streams are compound I, Q signals that have been modulated, therefore in some digital predistortion systems required to provide independent I, Q signals respectively, an extra I, Q demodulation process will be needed; however, most RF power amplifiers are produced by particular manufactures, so sometimes it is inconvenient to modify the design of the digital base band for adding the predistortion function. Therefore, a method that can directly use the compound I, Q signals in RF and IF to achieve complete predistortion compensation is desired.